Successful horse-breeding demands a special knowledge of horses, so far as concerns their external conformation, aptitudes for different services, and peculiarities and defects; and in its practical aspect it requires also a sound knowledge of horse-rearing and management, particularly of the young-stock, and of mares during pregnancy and parturition, and for some time after that event. Constant care and attention are likewise needed on the part of those entrusted with the carrying out of the details of breeding, in order to avoid accidents and ensure a satisfactory result.

The age at which horses commence to breed depends to some extent upon race peculiarities and external conditions, which have an influence in promoting precocity or retarding puberty. Well-bred animals are more precocious in this direction than those which are under-bred, and an abundance of rich stimulating food, easy labour, and comfortable surroundings expedite the development of the procreative faculties. The male and female horse are capable of breeding at two years of age, but instances are on record in which yearling colts and fillies have copulated successfully, and foals have been born before the parents were two years old.

The duration of the period during which procreation is possible is also dependent upon circumstances connected with breed, management, and surroundings. The stallion may continue potent until over thirty years of age, and mares have been known to produce foals when twenty-eight, thirty-two, and thirty-eight years old.

But it may be accepted as a rule that stallions and mares are at their best from four or five years, until they are about sixteen years old. When immature from youthfulness, or stale and decrepit from old age, the progeny of such animals cannot be expected to have the constitutional stamina or perfection of form of stock derived from parents in the bloom of life.

The mare is usually "in season "(ready to receive the stallion) from April to June, or even later, and the periods when conception is likely to take place during that time recur about once a fortnight or three weeks, and are very brief in some mares - only of two or three days' duration. The indications of this condition (aestrum) are generally well marked: the animal is usually irritable or sluggish, and less able to sustain severe exertion; the sensibility is increased, and the appetite is more or less in abeyance or capricious, and thirst is often present; there is a tendency to seek the company of other horses, especially males; attempts to pass urine are frequent, and there are spasmodic ejections of a whitish fluid, accompanied by movements of the vulva. While these symptoms continue, the mare will readily receive "service", and fecundation then most certainly occurs, - though it must be remarked that they often persist continuously in certain mares, and "service" does not allay them, neither does pregnancy result from such service, as they are mostly due to an abnormal condition of the ovaries. (See page 180 of this volume.)

When conception has taken place, these symptoms, as a rule, do not recur at these usual periods, and are not witnessed during the whole time of pregnancy, - though now and again instances are noted in which one or more of them are observed, and pregnant mares will sometimes accept the stallion, instead of repelling him, as is usually the case, though he rarely shows any desire to have intercourse with mares when they are in foal.

When conception has taken place, the signs of heat or rutting, as has been said, subside, and are not again noticed until after parturition; they reappear, however, very soon after that act has taken place, and it is believed that on the ninth day subsequent to foaling, the mare will be more successfully impregnated than at any other time.

With some mares impregnation does not take place readily, and this fault may be due to various causes, such as the animal being too old when tried for the first time, too fat or debilitated, etc, in which cases medicines which stimulate the generative functions, such as cantharides in very small doses, tonics, or stimulating food, may be of service. For other cases in which the cause is located in the organs of generation, the remedy to be resorted to will depend upon the character of the obstacle. The most frequent of the causes which hinder or prevent impregnation and produce sterility appears to be one of a mechanical kind - closure of the small opening (os) in the neck (cervix) of the uterus, leading to the interior of that important receptacle. This can only be ascertained by a manual examination, which discovers the opening into the uterus to be impervious, through contraction or alteration in structure of the neck of that organ. For very many of those cases the canal can be dilated by the fingers immediately before the mare is brought to the stallion; and great success has attended the employment of the india-rubber impregnation-tube, which is inserted into the canal before service, and withdrawn when that has been effected.

When impregnation has been successfully accomplished, certain changes are usually observed in the behaviour of the mare which lead to the supposition that such is the case. Perhaps the most notable indication is the disappearance of "aestrum" or "heat". It is ordinarily the practice to present a mare to the stallion nine days after she has foaled, this being the time at which, as has been already stated, conception is popularly believed to take place with most certainty. About a fortnight afterwards she is again presented, and generally in another fortnight a last trial is made, when if the animal refuses intercourse it is concluded that she is pregnant, especially if no unfavourable signs have been observed in the interval, such as a desire for the male.

In a short time, also, the majority of mares, if they have been irritable and restless previously, become quieter and more docile, if not absolutely torpid, and inclined to become fatter. Seldom is anything more noticed until pregnancy has advanced to the sixth or seventh month; so that though the question is often asked the expert as to whether a mare is in foal before that period, a reply in the affirmative is rather hazardous, and can only be based on the indications just alluded to, unless recourse be had to a manual examination per rectum or through the genital passage, a procedure which is not advisable in all cases.

But about the sixth or seventh month an attentive observer can generally detect an enlargement of the abdomen, more particularly on the right side, and movements of the young creature can also be seen in the region of the right flank, and most probably after the mare has been drinking cold water. The expert may also be able to hear the beating of the foetal heart.

From this time onwards the size of the abdomen gradually increases, and it becomes more pendulous and prominent, though the volume varies in different mares, the variation depending not only upon a difference in the size of the foal, but also upon the amount of the fluid which surrounds it in the uterus, this being much greater in some mares than others.

When the term of pregnancy is nearly completed, not only is the abdomen increasingly larger and more pendulous, but its upper part on both sides towards the spine begins to fall in, this hollowness being very marked immediately before parturition. A waxy matter also forms on the teats, and the udder becomes enlarged, this enlargement being generally coincident with the appearance of a thin discharge from the teats. The mare becomes sluggish, is readily tired, and seeks for rest and tranquillity, though the appetite, which has been greater during the later months of pregnancy than before, is usually unimpaired. A few days before foaling the croup sinks on each side of the root of the tail, and sometimes the hind-limbs swell slightly.